Radio-frequency transformer



Oct. 12, 1954 D. R. DE TAR RADIO-FREQUENCY TRANSFORMER Filed June 9, 1951 A T TORNE Y5.

Patented Oct. 12, 1954 UNITED STATES ATENT OFFICE Aladdin Industries, Incorporated, Nashville,

Tenn., a corporation of Illinois Application June 9, 1951, Serial No. 230,744

3 Claims. 1

This invention relates to an improved radiofrequency transformer particularly adapted for use as an antenna-coupling device in television receivers and other VHF and UHF radio receivers.

The present invention is an improvement on the radio frequency transformer disclosed and claimed in United States patent application, Serial No. 114,294 now Patent No. 2,591,081, filed by Harold T. Lyman and myself as joint inventors on September 7, 1949, and copending herewith.

Successful transformer design for VHF and UHF applications involves consideration of many objectives, some of which are difficult to realize simultaneously. One objective which is extremely desirable in an antenna-coupling transformer for radio-receiving apparatus intended for use over a wide frequency rangea television receiver, for exampleis a substantially constant input impedance at the antenna terminals.

Another desirable characteristic in any antenna-coupling transformer is low effective shunt capacitance across the secondary winding. This last-mentioned characteristic is highly important because the amplification of vacuum-tube ampli fiers in the VHF and UHF range is directly dependent on the quality factor of the tuned circuits associated with the amplifiers. That quality factor, in turn, is dependent upon the inductanceto-capacitance ratio. torily high inductance-to-capacitance ratio is easy at the lower frequencies, but in the VHF range and higher, it is hard to obtain a high inductance-capacitance ratio.

In the radio-frequency transformer heretofore invented by Harold T. Lyman and myself, and described and claimed in our earlier application heretofore mentioned, we disclosed a novel primary-coil construction having a substantially constant input impedance over a Wide frequency range and having also the inherent property of being insensitive to noise voltages and other electrical disturbances induced in the antenna transmission line. The primary-coil construction just referred to represents a great improvement over prior-art devices and has proved eminently satisfactory. That construction is characterized by a shield member physically interposed between the primary and secondary windings and completely surrounding the secondary winding except for a narrow axial break which prevents the shield from functioning as an electromagnetic shield without materially reducing its effectiveness as an electrostatic shield.

That shield, representing a relatively large Maintenance of a satisfacconducting area disposed close to the secondary winding, substantially increases the inherent shunt capacitance of the secondary coil. As a result, the aforesaid earlier transformer invented by Harold T. Lyman and myself was not characterized by particularly high inductance-capacitance ratio in the VHF and UHF range.

The primary object of the present invention, which is an improvement upon said earlier transformer, is to reduce greatly the inherent shunt capacitance across the secondary winding of a radio-frequency transformer wire retaining fully all the distinctive advantages which made our earlier transformer such a successful advance over the prior art.

Another object of the present invention is to provide, in a radio-frequency transformer adapted for VHF and UHF applications, a radiofrequency transformer having complete electrostatic shielding between the primary and secondary windings and having also the characteristic that it provides practically no transfer between the primary and secondary coils of energy derived from interference voltages induced in the antenna transmission line.

The special advantages of my improved radiofrequency transformer are realized with inexpensive circuit components and surprising simplicity of construction.

Other objects and advantages of my invention will appear as the specification proceeds.

I have illustrated in the accompanying drawing an illustrative embodiment of my invention.

In the drawing,

Figure 1 is a perspective view of a typical radio-frequency transformer embodying my invention;

Fig. 2 is a perspective view showing the appearance of my radio-frequency transformer after removal of the primary-coil structure;

Fig. 3 is a cross-sectional view taken along the line 3-3 in Fig. 1;

Fig. 4 is a transverse sectional view of the shielded pair of wires of which my primary coil may be formed; and

Fig. 5 is a schematic diagram showing the equivalent circuit of my radio-frequency transformer.

By reference to the drawing it may be seen that the radio-frequency transformer therein illustrated is wound on a conventional cylindrical form Ill, which may be made of plastic or other suitable material. The secondary winding consists of an electric coil H formed from flat metallic ribbon wound snugly on the form l0. In the illustrated embodiment, as may be seen from Fig. 2, I have shown the coil l I wound with a substantially higher turn density in an intermediate portion thereof than at either extremity of the coil. This type of construction is shown and claimed in the earlier application of Harold T. Lyman and myself hereinbefore referred to, and is not a part of the present invention. It is to be understood that the present invention may either be used with a variable-turn-density coil of the type shown or with a conventional, uniformly wound secondary coil. The secondary winding of the transformer will normally be tuned by means of a slidable conducting sleeve 13 carried within the hollow core of coil form I0. As may be recalled from said earlier application, the metallic sleeve l3, upon entering the interior of the secondary coil I I, magnetically short-circuits the portion of the coil which encompasses the sleeve and thereby reduces the effective inductance of said coil. The tuning sleeve l3 is shown herein merely for illustrative purposes and does not constitute a part of the present invention. It is to be understood that transformers made according to the present invention may employ tunin sleeves of the type shown. or, if desired, may be tuned in some other manner.

overlying the secondary coil 1 I is an insulating sleeve M, made of thin plastic, impregnated paper, or other suitable substance. The primary coil which is the principal feature of the present invention is wound over sleeve 24.

The primary coil proper is made up from a pair of parallel conducting wires, preferably copper or silver, enclosed within a tightly fitting conducting, flexible shield. Ordinarily this shield material, denoted is made in the form of metallic braid. It is characterized by veiy low electrical resistance and excellent shielding properties. In that figure the two wires enclosed within braid i5 are denoted respectively 16 and H.

The primary coil consists of two parts, one portion wound about one end of the secondary coil near the high-turn-density zone, and the other wound about the secondary coil over the lowturn-density zone on the opposite side of the highturn-density section. The number of turns in each portion of the primary coil is a matter of choice. Normally one turn of the braid, with its enclosed wires, around each section of the secondary coil will be sufilcient. As will be hereinafter shown. the resulting inductance is effectively a four-turn coil.

The two wires l6 and ET within the braid [5 are shown on the drawing, for purposes of ready identification, as having insulation of difierent colors, wire it having white insulation and wire ll having insulation of red, blue, or some other hue. As may be noted from Fig. l, the elongated portion of the primary coil extending axially along the coil form I!) and connecting the two portions of the primary winding is provided with a relatively large flat plate l8 formed from thin but highly conducting metal such as copper or silver. Plate l8 is continuously joined electrically to the braid i5 over the axially extending portion of braid [5 just mentioned; the connection is preferably by a continuous solder joint, although it can be effected by weldin or other means. As may be seen from Figs. 1 and 3, the two terminals of the primary coil are physically situated immediately adjacent the edges of plate l8. Wire I! is electrically joined, as by soldering, to plate I 8 at the point where wire I 6 extends out of braid I5 to form a terminal for external connection to the primary coil, and the other end of wire I6 is joined as by soldering to plate [8 at the other end of the primary coil, wherefrorn the free end of wire ll extends to form the other external connection to the primary coil.

If desired plate [8 may be formed with a narrow flange 48a approximately the width of the shielded cable comprising braid I5, and a second flange i8b, somewhat wider than flange 18a and situated at right angles thereto and also at right angles to the main surface of plate l3. Flange lGb provides astiffening or reinforcing means for plate l8. The main surface of plate [8 is coplanar with the axis of coil form I0, so that it is wholly edge-on to the secondary coil ll.

Study of the coil construction just described, and comparison with the construction shown in the earlier application of Harold T. Lyman and myself heretofore referred to, will reveal clearly the special advantages of the present construction. In both transformers the object is to provide, at high frequencies, a primary coil which is effectively shielded electrostatically from the secondary coil and effectively grounded at its midpoint. As may be seen from studying Fig. 1, transmission line currents in the present construction enter the primary coil on wire I, make two turns around the coil form it within the braid l5, pass across plate #8 to wire l1, make two more turns in the same direction through braid l5, and emerge on wire E? at the other end of the primary coil. The same efiect occurred in the earlier invention, the shield 38 serving in that device to carry the transmission line currents between the two conductors of the primary coil.

It will also be noted that in both constructions currents induced in the antenna transmission line by electromagnetic fields adjacent thereto come in simultaneously on wires IG and I1, make two turns around the coil form I!) in opposite directions, and are carried on to ground via plate l8. Thus the magnetic fields from such currents cancel and do not induce any energy in the secondary coil l I. At the same time, electrostatic coupling of such interference currents into the secondary coil I l is prevented by the Faraday shield which in the early application is element 38 and which in the present application is the braid l5.

The great improvement accomplished by the present invention is that the surface area of braid I5 exposed to secondary coil II is very small compared to the corresponding surface area of the shield 38 in the earlier construction. As a result, the capacitance between secondary coil 38 and ground is much smaller in the present construction than in the earlier one.

Mere substitution of braid [5 for the shield 38 formerly used, however, would not accomplish the present desirable and successful results. Those successful results are accomplihed by the cooperative action of the braid l5 and the plate 18. If the braid l5 were used alone, without plate IS, the desired objective of a midpoint ground potential for the primary coil could not be achieved, since the wires l6 and Il are joined to braid 15 at widely separated points, and the inductance of the length of braid connecting those junctions would result in sizable potential differences being generated therealong at the frequencies here under consideration.

It is a characteristic of conducting surfaces, as opposed to conducting filaments, that they possess very low inductance. Thus, by providing a largearea shield l8 continuously joining the portion of the braid l5 between the points of junction of the wires 16 and I1, I have provided a unipotential, grounded surface at the midpoint of the primary coil of my transformer. (It will of course be understood that in mounting my radiofrequency transformer on a chassis, the plate l8 will be conductively connected over a sizable portion of its surface to the grounded chassis. Flange i812 may be used for this purpose if desired.) It should further be noted that my novel construction provides complete electrostatic shielding of the primary coil without providing any closed loop in the braid l5. Thus the braid It does not cause any magnetic shielding, does not carry any eddy currents, and does not add any appreciable loss factor to the transformer.

The electrical effect of the novel construction just described is brought out clearly in Fig. 5. As may be therein seen, the construction produces a transformer having a center tapped primary winding, with the center tap held at all times at true ground potential, the primary coil as a whole being completely shielded electrostatically from the secondary coil. At the same time, the capacitance between the electrostatic shield and the secondary coil is held to a minimum. It has been well known that all these characteristics are highly desirable, but their physical realization in a transformer adapted for use at frequencies above 100 megacycles per second has been impossible prior to the present invention.

While I have in the present specification, for illustrative purposes only, described in detail, a single embodiment of my invention, it will be understood by persons skilled in the art that numerous changes may be made therein without departing from the spirit of my invention. It is accordingly my desire that the scope of my invention be determined primarily by reference to the appended claims.

I claim:

1. In a radio-frequency transformer having an elongated secondary winding, an electrostatically shielded, center-tapped primary coil formed from a cable comprising a pair of insulated wires and a metallic sheath closely overlying the same, said coil having two relatively short helical portions joined by a substantially axial portion, a conducting plate of relatively large surface area joined mechanically and electrically to the sheath substantially continuously along said axially extending portion thereof, said plate having surface portions extending adjacent the ends of said coil and being oriented substantially edgeon to the helical surface defined by said coil and extending alongside said axially extending portion, an electrical connection between one of said wires and said plate at one end of said coil, and an electrical connection between the other of said wires and said plate at the other end of said coil.

2. In a radio-frequency transformer having a secondary winding, .9. primary coil comprising a pair of insulated wires, an electrically conducting sheath closely overlying the wires, said wires and sheath being formed into an elongated helical coil of spaced turns, said wires emerging from said sheath at each end of said coil, a conducting plate of relatively large surface area extending along the longitudinal axis of said primary coil and conductively connected to said sheath continuously along an intermediate portion thereof, defining a substantially uni-potential surface between the respective ends of said coil without defining any closed circuits for circumferential flow of currents in said sheath and plate, an electrical connection between one of said wires and said plate adjacent one end of said coil, and an electrical connection between the other of said wires and said plate adjacent the other end of said coil.

3. Apparatus according to claim 2 wherein the plate is oriented substantially edge-on relative to the secondary winding, whereby the capacitance between the plate and the secondary coil is minimized.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,526,549 Frease, Jr. Oct, 17, 1950 2,591,081 Lyman et al Apr. 1, 1952 

